1. Field of the Invention
The present invention relates to a vibration actuator with a substantially rod-shape vibration member.
2. Related Background Art
As a conventional ultrasonic wave motor, a motor which produces a bending vibration in a ring, and drives a movable member by a frictional force has been put into practical applications in, e.g., an AF mechanism for a camera. However, since such a motor has a ring shape, it requires unexpectedly high cost as a unit including a compression mechanism, and is disadvantageous in terms of cost in a motor application which does not require a hollow structure. Thus, as a motor which has a solid structure and facilitates an arrangement of a compression system, Japanese Laid-Open Patent Application No. 4-117185 has been proposed.
FIG. 2 is a sectional view showing main part of an ultrasonic wave motor according to the proposal. Hollow disk-shape piezoelectric elements a1 and a2 are polarized to (+) and (-) in the direction of thickness to sandwich their central lines therebetween, and are arranged to be positionally shifted by 90.degree.. Upper and lower vibration members b1 and b2 are respectively arranged on opposite sides of the piezoelectric elements a1 and a2. A radially constricted portion b1a is formed in the upper portion of the upper vibration member b1 near a contact portion A. When electrical signals having a phase difference therebetween are respectively applied from a driving circuit (not shown) to the piezoelectric elements a1 and a2, bending vibrations, which temporally have a predetermined phase difference, are generated in different planes of the vibration member. In this manner, the contact surface of the vibration member makes a rotary motion. Note that this principle is known to those who are skilled in the art, and a detailed description thereof will be omitted.
A vibration member side contact portion A in FIG. 2 is used for driving a movable member dA. The movable member dA has a contact portion dAA which is in pressure contact with the contact portion A of vibration member b, and receives a driving force.
Recently, optical devices and OA devices such as printers have been required to be miniaturized. However, in the prior art, when a rod-shape ultrasonic wave motor undergoes miniaturization, since the resonance frequency of a mode utilized in driving decreases in inverse proportion to a square of the total length in accordance with a formula .omega.n=Knd(.sqroot.E/.rho.)l.sup.2 (E=Young's modulus, .rho.=density, l=total length, d=diameter, Kn=constant), the following problems are posed:
(1) When driving control is executed by feeding back the phase difference between a detection signal detected by a sensor piezoelectric element and a driving signal, since the detection signal and the driving signal have high frequencies, the phase difference cannot be accurately detected, and control cannot be executed with high accuracy. PA1 (2) In the ultrasonic wave motor, since the entire surface of the movable member is brought into contact with the driving surface of the vibration member, the contact portion of the movable member is provided with elasticity (contact spring portion). The displacement distribution of the vibration member upon driving has a sinusoidal pattern, and an exciting vibration force applied to the contact spring portion which is in pressure contact with the vibration member is half-wave rectified, as shown in FIG. 3.
At this time, the exciting vibration force components include many components of a frequency as high as twice the driving frequency, and if the natural frequency of the contact spring portion is present near this frequency, then the amplitudes of these components are amplified, thus disturbing regular rotation and generating sound noise during a driving operation.
In order to avoid this phenomenon, in the prior art, the natural frequencies of all modes of the contact spring portion are set to be higher than a frequency twice the driving frequency.
However, as described above, when the rod-shape ultrasonic wave motor is miniaturized, if miniaturization is performed while maintaining a similar shape, since the driving frequency increases in inverse proportion to a square of the axial length, the natural frequency of the contact spring portion must be inevitably increased. However, the present inventors have discovered out that when the resiliency of the contact portion becomes too high, the contact portion cannot follow the waviness which is always present on the driving surface, and the motor output is undesirably lowered.